Electric strain gage



June 23, 1964 v. L. SULLIVAN ETAL 3,138,775

ELECTRIC STRAIN GAGE Filed July 19, 1961 INVEN T ORS [fe/da L. Jul/ivan J'ames arseg United States Patent O 3,138,775 ELECTRIC STRAlN GAGE Velda L. Sullivan, Ashland, and .lames Dorsey, Winchester, Mass., assignors to Baldwin-Lima-Hamilton Corporation, a corporation of Pennsylvania Filed July 19, 1961, Ser. No. 125,240 1 Claim. (Cl. 3138-2) This invention relates generally to electrical resistancetype strain gages and more particularly to such a gage that is adapted for high temperature applications.

Many different types of strain gages proposed for use in high temperature work have included a single strainsensitive filament connected between a short lead and a long lead which extends for the length of the gage parallel to the gage filament. Such mono-filament gages have been difiicult to manufacture with consistent characteristics. One reason is that thermal expansion of the long lead seriously affects the accuracy.

It is an object of my invention to provide an improved electrical resistance bonded type strain gage of the foregoing type in which the gage is substantially free of adverse effects of thermal expansion of the long lead.

Other objects and advantages will be more apparent to those skilled in -the art from the following description of the accompanying drawing which is a greatly enlarged longitudinal sectional View of our improved gage.

It will be understood that the gage disclosed herein is particularly adapted for sub-miniaturization so that the gure in the drawing is enormously magnified as compared to the actual gage. In the particular embodiment of the invention disclosed herein for purposes of illustrating one particular form among possible others that the invention might take in practice, we have shown short metallic tubular lead 1 and an elongated metallic tubular lead 2 curved reversely upon itself as at 3. An electrical strain-sensitive filament 4 lying parallel to lead 2 is preferably of the line wire type and extends into the interior of lead 1 and of reverse lead portion 3. The filament is held by swaging the tubular ends as at 5 and 6. A silica tube 7 extends over the long tubular lead 2 and serves not only as a spacer to prevent the filament 4 from being electrically shorted by contacting the lead 2 at a point intermediate of its length but in addition prevents external strains being transmitted to the long lead 2. To accomplish this the ceramic coating 8 stops short as at 9 of the end 10 of sleeve 7 thereby allowing the external extension 11 of the conductor lead 2 to be entirely free of the ceramic coating or cement 9. The reverse end 3 of the conductor lead 2 is imbedded in the ceramic coating as is also the case with the end of lead 1.

As a result, the filament 4 will have strains transmitted thereto only between the curved end 3 and the end S of the short lead. The remaining length of the center lead 1 will not have any strain transmltted to 1t from the speclmen,

as would be the case if any part of the external portion 3,138,775 Patented June 23, 1964 11 of the lead were bonded to the specimen surface as has heretofore been the case. Hence the elongated lead is adapted to have free lengthwise movement arising from thermal expansion or other causes without in any way changing the length of filament 4 and its electrical resistance.

As an illustration of the practical size of strain gages made in accordance with our invention the inside diameter of sleeve 7 is .007 and the outside diameter of center lead 2 is preferably .005. In spite of this extreme miniature size our improved construction readily prevents the center lead from adversely affecting the accuracy of response of the gage filament 4. Heretofore, gages of this so-called mono-filament type have been so unpredictable in their operation as to seriously reduce their usefulness, whereas my improved construction permits a high degree of consistency in manufacturing a mono-filament gage of desired uniform characteristics.

It will, of course, be understood that various changes in details of construction and arrangement of parts may be made by those skilled in the art without departing from the spirit of the invention as set forth in the appended claims.

We claim:

A strain gage having an elongated lead, one end of which extends to a point externally of the gage for connection to a circuit and the other end of which has a terminal, an electrical strain-sensitive filament lying substantially parallel to said elongated lead and having one of its ends connected to the terminal of said lead, a second lead having a terminal to which the other end of the filament is connected, a sleeve of insulation material, the elongated lead extending entirely through said sleeve and beyond both ends thereof to provide the lead terminal and the external connection, a casing of insulation bonded to the lead terminals and sleeve and to the filament so that they all form a unitary structure, said casing being free of the remainder of the elongated lead so that when the gage casing is secured to a specimen surface whose strains are to be measured the lead is free lto thermally expand longitudinally within the sleeve, whereby the filament is subjected substantially solely to strains transmitted from the specimen surface to which the gage is attached, said combination being further characterized in that the elongated lead is a tube and its terminal is reversely curved to lie substantially parallel to the elongated p0r- .tion of the lead, and the second lead also being a tube whose terminal is substantially in alignment with the reversely curved terminal, the filament extending straight into the tubular portions of the aligned leads which are swaged onto the filament.

References Cited in the file of this patent UNITED STATES PATENTS 

